The modelling of permanent deformations of pavements is more complex than the modelling of the resilient behaviour because it is necessary to simulate the response of the pavement to large numbers of load cycles (typically 105-106 cycles), with variable loading and environmental conditions.
A programme for the prediction of rutting of low traffic pavements, called ORNI, is also implemented in the finite element code CESAR-LCPC (El Abd et al., 2005; Hornych & El Abd, 2006). To determine the permanent deformations due to large numbers of load cycles, this programme proposes a simplified approach, based on a separate calculation of the elastic response and of the plastic strains. It comprises 3 steps:
i) The first step consists in calculating the resilient response of the pavement, for the different loading conditions considered (different types of loads, different temperature, etc…). The resilient response is calculated in 3D, using the programme CVCR.
ii) Then, the resilient stress fields are used to calculate the plastic strains produced by the successive application of the different loads. The permanent strains are calculated locally, at different points in the pavement structure, in 2D (in the plane (0,y, z) perpendicular to the direction of displacement of the load).
iii) Finally, the third step consists in calculating the displacements in the pavement structure. The plastic strains being calculated locally, at different points, do not derive from a displacement field. It is thus necessary to determine the total strains, ensuring their continuity and integrability, and the corresponding displacements.
Two permanent deformation models are implemented in ORNI: the empirical model of Gidel et al. (2001), and the elasto-plastic model of Chazallon (Chazallon et al., 2006). These models have been described in Chapter 9.
